iNKT cells are now recognized as powerful cytokine producing cells that are activated by either self or foreign lipid antigens presented by CD1d (signal 1) in combination with cytokine driven second signals. The structure of the lipid antigens, nature of the APC and type of cytokine are all peripheral cues that influence the effector cytokines they produce. iNKT cells may take on Th1-like, Th2-like and Th17-like responses determined both by their thymic differentiation and by the peripheral signals they encounter during activation. Their development in the thymus is dependent on the up-regulation of the transcription factor, PLZF, considered a 'master regulator' of the lineage. This transcription factor is responsible for expression of the effector/memory phenotype that enables iNKT cells to be 'poised' for rapid secretion of effector cytokines, without requiring priming. Recognition by th semi-invariant TCR enables much of the population to respond, all at once, to the same antigen. Thus, iNKT cells are considered 'innate' T cells. Here, we propose to characterize a newly identified subset of iNKT cells that is remarkably different from those described before. iNKT cells localized in adipose tissue do not display the Th-1, Th-2 or Th-17 phenotype of other iNKT cells, but instead primarily produce IL-10 and IL-2. They regulate inflammation in adipose tissue by driving macrophages to the M2 phenotype via IL-10 and by regulating the number and the function of Tregs in adipose tissue via secretion of IL-2. Their unusual cytokine skewing is part of a larger underlying program that we found to be characterized by the lack of expression of PLZF and up-regulation of the transcription factor E4BP4. We show that E4BP4 drives their IL-10 secretion and down-regulates PLZF expression. In Aim 1, we will define the molecular mechanism by which E4BP4, a negative regulator of transcription, controls PLZF expression by determining if it binds to the PLZF promotor or induces chromatin changes influencing the PLZF locus. We also show that exposure of non-adipose iNKT cells to adipose tissue activates them via TCR recognition, up-regulates E4BP4 and turns on a new underlying transcriptional program. We show that adipose tissue lipid antigen extracts and adipose tissue-derived soluble factors both activate iNKT cells. Thus, we propose to purify and identify the lipid antigens in adipose tissue that activate iNKT cells (Aim 2) and identify the soluble factors from adipose tissue that activate iNKT cells (Aim 3) and combine these adipose-derived activation cues with CD1d expressing adipose APCs to reconstitute adipose-driven iNKT cell activation and skewing in vitro. Together, these aims will define the environmental (adipose tissue-selective) activation cues and transcriptional program that drives a new innate regulatory iNKT cell subset with important implications for adipose tissue inflammation, obesity, and type II diabetes.